Frequency hopping (FH) is a well known spread spectrum technique. Frequency hopping can be used as a multiple access technique in order to share a communications resource among numerous user groups. Since a user group typically employs a unique spread spectrum signaling code, (e.g., frequency hopping set) privacy between individual user groups is easily established. Moreover, radio interference such as CO-channel interference, adjacent channel interference, and Raleigh fading is greatly reduced.
Local area networks use spread spectrum signaling techniques where a series of user devices, which are typically battery powered, communicate with the access point (AP) which functions as a central controller and may also act as an information relay device. A radio transceiver or wireless adapter within each user device and within the AP provides the radio communication function. The AP typically provides network services such as synchronization, authentication, wireline access, packet relay between user devices, and the like. Communication between user devices may be accomplished directly between user devices or may be accomplished by relaying information from a source user device to the AP and back to a destination user device. The group of user devices and the corresponding AP are referred to as a microcell.
Microcells may also exist that do not have an AP. Such systems are referred to as ad-hoc microcells. For example, a microcell may consist of several portable computers that form a temporary network established for the duration of a meeting in a conference room. In this case, one of the user devices is designated as a master device and is responsible for synchronization between the other user devices.
In a frequency hopping LAN, all devices within one microcell share the same hopping sequence. Each user device changes receiver frequency in unison with all other user devices within the microcell and in unison with the AP such that the change in frequency is virtually transparent to the user devices. The time spent transmitting user information or channel control information (synchronization information) on each individual frequency of the hopping set is referred to as a dwell. The dwell is typically long enough to allow several transmissions of data, referred to as packets, to be transmitted to or from the user devices or the AP.
Many office environments have several microcells operating in close proximity to each other and may be used to provide extended coverage over a large area. In such a situation, microcells must not interfere with neighboring microcells and must facilitate transparent link transfers therebetween. When a user device seeks or requires access to the AP of a different microcell, a link transfer or "hand-off" must be performed between the two microcell controllers (i.e., APs). In most cases, however, neighboring microcells employ completely different spread-spectrum signaling codes or hopping sequence sets. As a consequence, acquisition and synchronization are formidable challenges during a hand-off.
If a hand-off is too slow the network may experience various time-outs which may in turn cause the user's session to be dropped, resulting in disconnection from the network. Some systems may even lock-up or experience serious system performance degradation when this occurs.
Typical wireless LANs and devices associated therewith may use a channel access protocol such as carrier sense multiple access (CSMA) to determine a time during which to transmit information, as is known in the art. When two devices transmit at the same time, a collision occurs and neither transmission is successful. The purpose of the CSMA protocol is to allocate the frequency channels fairly among the devices to maximize information throughput.
Other LANs use a channel access protocol such as carrier sense multiple access with collision avoidance (CSMA/CA) to determine a time during which to transmit information. This technique allows the devices to "sense" if a channel is in use prior to transmission. If the channel is in use or is busy, transmission is deferred to another time according to specific collision avoidance algorithms.
Typical LANs utilize fixed (i.e., identical) dwell times for each microcell within the system. Fixed dwell times promote efficient handoffs. In addition, when dwell times are set long, the overhead associated with transmitting channel synchronizing information is reduced as is data packet fragmentation near the end of the dwell. When dwell times are set short, the short-term effect of frequency-selective fading, multipath interference, and other forms of interference are reduced. Despite these standard approaches, it is nevertheless desirable to allow different microcells within the same system to use different dwell times. Unfortunately, currently available LANs are not designed to handle the complexities of a microcellular system employing different (i.e., configurable) dwell times.
It would be extremeley advantageous therefore to provide a method for the fast acquisition and synchronization between a user device and a new AP employing a new frequency hopping set. It would also be advantageous to provide a method where traffic control information and synchronization information is provided on a frequency different from the frequencies in which data is transmitted. Such an advantageous system must handle the complexities of configurable (i.e., different) dwell times for the various microcells within the system.